The present invention relates to the manufacture of polymeric elastomeric materials (elastomers), e.g., those in an uncured state used in the manufacture of rubber products such as tubing, belts, tires, sheeting, etc., by subsequent cure or vulcanization of formulations embodying the elastomeric materials. More specifically, it relates to a method for making melt-coated, free-flowing elastomeric pellets to improve the handling of the same in packaging, shipment and use.
Most elatomers are normally shipped and used as bales, especially those which are tacky or subject to cold flow. This practice has several disadvantages particularly as it relates to rubber. The material cannot be transported in bulk, the bales must be cut or milled on expensive, large scale equipment and the bales cannot be preblended with other materials. The result is that these materials are expensive to handle and ship. More important, elastomers have not reached their full sales potential in many market areas. They have either been restricted to batch rather than continuous processes in normal applications, or a separate step of grinding is required prior to the continuous process. Plastics blending applications have been particularly plagued with these problems, since the plastic component of the blend is in pellet form. It has been considered desirable for some time to obtain elastomers in pellet form. The difficulty encountered for most elastomers has been that the pellet surfaces adhere to each other or in a very short time flow together into a solid mass.
Numerous attempts have been made to overcome these disadvantages. Dusting with inorganic materials, e.g., clay, talc and the like, allows maintenance of discreet pellets for short periods of time. Slightly better results have been attained by dusting with selected organic materials such as hydrocarbon waxes (British Pat. No. 901,644) or powdered solid polyethylenes and polypropylenes (British Pat. No. 928,120). However, all of these methods are useful only for relatively short periods of time. On long-term storage, because of the discontinuity of the pellet coating, the pellets will still flow together into a solid mass.
Another approach to the problem, yielding somewhat better results, has been to blend into rubber a crystalline type of polymer such as polyethylene or polypropylene or copolymers thereof, during the rubber finishing operation. This renders the product non-tacky and non-flowable and, therefore, suitable for pelletization. Disadvantages of this type of operation are that the resulting pellets, while relatively free flowing, contain a maximum of only about 50% rubber with the crystalline polymer intimately admixed therewith. Such a high percentage of irremovable crystalline polymer makes this type of pellet disadvantageous for many purposes.
A further approach to the problem relates to emulsion coating techniques in which rubber is either dipped in an emulsion of coating material in a solvent or the emulsion is sprayed on pellets of the rubber. In either case, the emulsion-coated rubber must be dried as, for instance, in a fluidized bed through which hot air is circulated to evaporate the solvent. However, emulsion coating techniques are undesirable because they require the use of solvents. The cost of the solvents add to the overall cost of the coating process. Also, if the solvent is to be recovered, extra equipment for a solvent recovery system must be purchased, operated and maintained. If hot air is to be used to evaporate the solvent from the emulsion-coated rubber, the expense of heating the air is also incurred. Also, significant deficits can be incurred due to lost process time, because the coating and/or drying steps are typically batch operations.
Melt-coating methods for producing free-flowing rubber pellets have also been suggested. According to U.S. Pat. No. 3,669,722 to Bishop, heated melt-coating material and rubber to be coated are fed as continuous films from separate extruders to a common coating die. A continuous melt-coated rod of rubber issues from the coating die, is cooled in a cooling liquid bath, and is then cut into pellets by a pelletizer. Not only is this melt-coating system seen to add excessively to the overall cost of rubber manufacture, but it is also seen to have limitations with respect to efficiently producing large quantities of coated rubber pellets. According to British Pat. No. 1,322,623 to Kresge et al, pellets of the rubber to be coated are first heated to a temperature higher than the melting point of the coating material. The pellets are then contacted with the coating material which is preferably in the form of fine powder. The heated pellet fluxes the coating material on the surface of the pellet to form a substantially continuous film. The hot, coated pellet is then cooled.
While such melt-coating is highly preferred for producing free-flowing pellets of rubber, a commercially attractive method for producing such pellets has not yet been developed. Significant problems faced in developing a suitable method relate to lost production time or economic incentive in heating the coating material to a temperature above its melting point, achieving substantially complete coverage of the rubber pellet with solid coating material before it is melted, preventing the pellets from adhering to each other while the coating is molten, and cooling the melted coating to a temperature below its melting point to set the coating on the pellet.
With respect to supplying enough heat to the coating material to get its temperature above its melting point, the above-noted patent to Bishop provides a separate extruder for supplying melted coating material to a coating die. However, Kresge et al does indicate that if the coating material is to be applied as a fine powder to pelletized rubber, a normal extrusion process for making the pellet may generate sufficient heat to melt the coating material.
With respect to techniques for applying the coating material to the rubber, Bishop applies the same in preheated melt form to a continuous rod of rubber which is subsequently cooled and cut into pellets. Kresge et al is silent on this point.
With respect to cooling the melted coating once it has been applied to the rubber, Bishop suggests the use of a bath of cooling liquid. Kresge et al suggests the use of a water bath or air cooling.
The present inventor has discovered a method for producing melt-coated, free-flowing pellets of elastomeric material which method is seen to be commercially attractive. Broadly, the present inventor has discovered that pellets of elastomer such as those produced in the course of extrusion or extrusion drying can be continuously coated by the use of novel concepts built into a pneumatic conveying system. Basic steps of a method in accordance with the present invention comprise:
1. mixing coating material with at least one stream of carrier gas to form at least one coating stream, the carrier gas having a temperature below the melting point of the coating material;
2. contacting at least one pellet to be coated having a temperature above the melting point of the coating material with the coating stream so as to form at least one layer of substantially melted coating material on the rubber pellet; and
3. cooling the resulting melt-coating to a temperature below its melting point temperature.
The pellets are introduced into the coating stream so as to create at least one "zone of interference" in which a relative velocity difference exists between solid particles of coating material and the pellets for some distance along the pneumatic conveying system, that is, the solid particles of coating materials are traveling faster than are the pellets. This hesitation of the pellets before they attain the speed of the coating stream permits the solid coating material to bombard the pellets, substantially covering the pellet surfaces with particles of coating material. The heat contained in the pellets will then cause at least partial fusion of the solid particles permitting the formation of a substantially melted coating on the pellets. In turn, the temperature and heat content of the carrier gas is low enough to then solidify the thus-melted coating. By causing the pellets to flow co-currently with the coating stream from the zone of interference to a downstream zone, agglomeration of the pellets is avoided as the co-current carrier gas tends to spread the pellets out.
It was discovered that in a pneumatic conveying system such a method could be practiced in which the total energy and heat transfer characteristics of the pellet, the carrier gas and the coating material was sufficient to melt-coat the pellet and then cool the resulting melt-coating to a temperature below its melting point. In practicing methods in accordance with the present invention, the relative heat transfer rates between the pellet and the coating material and the pellet and the carrier gas should be controlled to ensure that step 2 is accomplished before step 3, which opposes step 2, quenches the system. Thus, the temperature of the carrier gas should not be so low as to cause the cooling of the pellet before a substantially continuous coating is fused to the pellet surface. Similarly, the temperature of the pellet should be high enough to fuse a substantially continuous layer of coating material around the pellet before the carrier gas solidifies the fused coating. Having the benefit of the present disclosure, the artisan could easily accomplish these objectives using well-known thermodynamic calculations as illustrated below in EXAMPLE 1.
In practicing methods in accordance with the present invention, melt-coated, free-flowing pellets of elastomeric material can be made in a continuous manner with essentially zero process inventory, and using only a minimal amount of excess energy above that required to melt the coating material. It is preferred that the energy is not specifically generated to accomplish the coating, but is the same energy used to make the extruded pellet.
It is common in the published literature in the area of manufacturing rubber pellets to see batch-type coating operations in which the pellets are recirculated or held in place until they are adequately coated. This creates an undesirable "inventory" of rubber pellets while they are being coated. See, for example, U.S. Pat. No. 3,669,722 to Bishop wherein rubber crumb is fed batchwise to a fluidized bed in which the rubber pellets are continuously recirculated until coated.
For general background relative to coating techniques for coating particles, both elastomeric and non-elastomeric, reference is made to the following:
U.S. Pat. No. 2,895,939 discloses making non-agglomerative rubber pellets by dusting the same with an impalpable resinus vinyl aromatic polymer dust.
U.S. Pat. No. 2,059,983 discloses a system for melt-coating preheated particles of inorganic abrasive material with thermo-setting resin to render abrasive particles which do not cohere to one another, but which on subsequent heating can be pressed together and made to cohere. The particles are preheated in a jar to a temperature above the melting point temperature of the coating material, but below the melting point of the particles to be coated, and then caused to cascade downwardly over a mixing baffle and then counter-currently to an upwardly traveling stream of cooling air in which is entrained the coating material.
U.S. Pat. No. 3,241,246 discloses a method and apparatus for simultaneously drying and dust-coating rubber crumb in a fluidized bed. A non-melting coating material, such as talc or carbon black, is mixed with the drying medium, superheated steam, and then dusts the rubber pellets in the fluidized bed. The rubber pellets are made inside the fluidized bed apparatus using an extruder die/cutter arrangement. The rubber pellets are retained in the fluidized bed and then overflow into a box outlet for further processing.
U.S. Pat. No. 3,241,520 discloses particle coating apparatus in which the particles are recirculated and spray-coated with an emulsion coating until sufficiently coated. A heated gas stream causes the particles to recirculate and dries the coating. FIG. 13 illustrates a semi-continuous operation in which seven spray-coating systems are arranged in series.
U.S. Pat. No. 3,253,944 shows a similar particle coating system in which particles are recirculated through a spray of emulsion coating until sufficiently coated.
U.S. Pat. No. 3,503,778 discloses a method of melt-coating a continuous web of substrate using a plastic powder.
U.S. Pat. No. 3,528,841 discloses a method for reducing the tackiness of polymer pellets by coating the same with microfine (average size less than 10 microns) polyolefin powder by dispersion techniques, tumbling, electrostatic transfer or airveying.
U.S. Pat. No. 3,687,699 discloses the use of talc as a dusting agent to render tacky pellets less tacky. The dusting is done simultaneously with both the cutting of the pellets and the cooling thereof by the combined use of a water spray and flowing air.
Great Britain No. 928,120 discloses coating polymer pellets with polyethylene powder using tumbling or dripping techniques. The pellets can be cold or hot (120.degree.-150.degree. F.) when coated.
Great Britain No. 1,105,680 discloses a continuous process for wax-coating granular thermoplastic using a melt-coating technique.